1. Field of the Invention
The present invention relates to a center mechanism for a tire vulcanizing machine, and more particularly to a center mechanism for a tire vulcanizing machine of the type in which a bladder is stretched upon the loading and unloading a tire.
2. Description of the Prior Art
As a bladder to be mounted in an automatic tire vulcanizing machine, generally classified, two types of bladders having a cylindrical shape and a mushroom shape, respectively, have been known, and normally the following two types of operation methods have been practically used:
(1) One is a method of housing a bladder by twice folding and drawing it into a housing cylinder upon loading and unloading a tire as disclosed, for instance, in Laid-Open Japanese Patent Specification No. 51-17282 (1976), and according to this method, regardless of whether it is a mushroom-shaped bladder or a cylindrical bladder, a bladder having a smaller upper opening diameter than a lower opening diameter can be mounted for use.
(2) The other is a method of only stretching of a bladder upon loading and unloading a tire as disclosed, for instance, in Laid-Open Japanese Patent Specification No. 57-399040 (1982), and according to this method, solely mounting and use of a cylindrical bladder is contemplated.
Furthermore, a method of housing a bladder by drawing it into a housing cylinder while keeping it stretched, has been proposed as a third method, but it involves structural problems (see the subsequent description), and it has not yet been practically used in general.
The above-described operation methods in the prior art involved the following problems. That is, especially in the case of vulcanizing a radial tire, in view of uniformity of a tire it is desirable to use a bladder having an outer shape close to an inner shape of a vulcanized tire, and hence the use of a mushroom-shaped bladder is abruptly increasing, but according to the above-described method (1), as a result of the popularity of radial tires, an inner diameter of a tire, that is, an outer diameter of a bladder has become large as compared to a bead diameter, and there occurred a problem in that a life of a bladder became short in relation to an allowable housing cross-sectional area of a housing cylinder.
Also, according to the above-described method (2), when a green tire is to be inserted, in the case of a radial tire or the like, upon contracting a height of a bladder it is necessary to introduce shaping steam into the bladder and to make the bladder guide the a loader shoe and the like, because the bladder would be twisted if the inside of the bladder is held at vacuum, and therefore, the method (2) involves problems in that a bladder is liable to have its vertical symmetry lost due to an initial period of creep, also that upon taking out a vulcanized tire, due to its positional relationship to an upper clamp ring for a bladder, an internal grip type unloader cannot be used, but a forklift type unloader or an external grip type unloader is necessitated, hence a cycle time is prolonged and in some cases even a carry-out fault would arise.
Furthermore, according to the above-described third method, while the above-mentioned problems of the method (2) are to be resolved theoretically if provision is made such that upon loading and unloading a tire, an upper clamp ring of a bladder may be lowered to a position below the upper bead of the tire, in practical use it becomes impossible to bear a depressing force of a lower clamp ring of a bladder during vulcanization of a tire via a lower bead ring, hence a stopper would be additionally provided, but there still remain problems that in view of an available space it is difficult to insure a mechanical strength, and that a pit for installing a tire vulcanizing machine becomes extremely deep.